Functional elements of the above named type in the form of fastener elements have been sold by the company Profile Verbindungstechnik GmbH & Co. KG in various forms for many years under the designations SBF, SBK, RSF and RSK. The elements themselves and also a method for the attachment of the elements and the die buttons that are used are described in detail, among other things, in the German patents DE 3447006 C2, DE 3446978 C2, and DE 3835566 C2.
Fastener elements of this kind are either introduced in self-piercing manner into the sheet metal part so that the element has a piercing and/or rivet portion. They can, however, also be introduced into pre-holed metal sheets, with the pre-holing frequently taking place using a so-called preceding hole punch which pierces the sheet metal part directly prior to or during the attachment of the fastener element. This possibility only exists for functional elements which are formed as hollow body elements, for example nut elements, because the holed punch must pass through the fastener element.
The possibility also exists of using functional elements which are provided with a piercing rivet portion in such a way that the sheet metal part is holed by the piercing and/or rivet portion prior to or during the attachment process. When realizing the functional element as a bolt element, the slug which arises in this way is frequently clamped within the tubular rivet portion where it can also exert a support function for the rivet bead. For hollow body elements the piercing slug is normally removed by means of a follow-up ejection pin which is passed through the hollow body element.
Irrespective of whether the corresponding functional element is self-piercing or is used with a pre-holed metal sheet, the piercing and/or rivet portion has to perform not inconsiderable deformation work on the sheet metal part because it has to ensure that the sheet metal part is deformed into a tubular collar in the region of riveted connection. The piercing and/or riveting section is subsequently deformed radially outwardly around the free-standing end of the tubular collar in order to form a recess for receiving the tubular collar, the recess being U-shaped in cross-section. In this connection, the tubular collar is frequently folded back on itself so that a ring-like fold of the sheet metal is present within the U-shaped recess formed by the beaded over rivet portion.
The requirements placed on the piercing and/or riveting section, irrespective of whether these operate with or without pre-holing are thus, on the one hand, to have sufficient stability to be able to perform the required deformation work and, on the other hand, after the formation of the rivet bead, to achieve sufficient strength in the region of the rivet bead that the strength of the connection that is aimed at is ensured. This is necessary for the firm attachment of an article to the sheet metal part. For a fastener element this signifies that the strength of the piercing and/or riveting section must correspond to the strength class of the screw. For the attachment of a fastener element to the sheet metal part the piercing and/or riveting section may not buckle. On the other hand, the deformability must be present so that the piercing and/or riveting section can be reshaped to form the rivet bead which is U-shaped when seen in radial cross-section.
These contrary requirements lead to a situation in which the radial wall thickness of the tubular piercing and/or riveting section must have a clear association with the effective size of the fastener portion. The term “effective size of the fastener portion” will be understood to mean the strength which is required from the fastener element per se.
Even if the said fastener elements have been successfully used in practice for many years, they nevertheless have certain disadvantages, namely that the attachment of the elements, irrespective of whether this takes place with or without pre-holing, can lead to the formation of small chips which, in the course of time, can be trapped in the region of the die buttons or elsewhere in the tools and presses that are used and can lead to jamming and deformation of sheet metal parts that are being processed, which are entirely undesired. The cause for these small chips is extremely difficult to establish because one cannot visualize the creation of the chips in a large press; since the working region consists of solid metal and there is no possibility so to say to provide observation windows which would make it possible to consider the manufacturing process without significantly impairing the course of the manufacturing process.
With the design of the piercing and/or riveting section which is described and claimed in European patent 1430229 one succeeds in substantially suppressing the formation of the chips. Nevertheless, a situation can arise in which the rivet connection in the region of the turned over end of the rivet bead or in the region of the sheet metal edge of the backwardly folded sheet metal part does not always appear perfect and it can above all transpire that the folded back part of the sheet metal part is cut into or pressed in crescent-like manner by the turned over end of the rivet portion so that parts of the sheet metal project as a chip from the rivet bead or can indeed by fully parted and cause problems in the tooling and leave marks in the sheet metal part.